Re: [PATCH RFC 00/77] Re-design MSI/MSI-X interrupts enablement pattern

Ben Hutchings
2013-10-07T20:46:06Z

On Tue, 2013-10-08 at 07:10 +1100, Benjamin Herrenschmidt wrote:
> On Mon, 2013-10-07 at 14:01 -0400, Tejun Heo wrote:
> > I don't think the same race condition would happen with the loop. The
> > problem case is where multiple msi(x) allocation fails completely
> > because the global limit went down before inquiry and allocation. In
> > the loop based interface, it'd retry with the lower number.
> >
> > As long as the number of drivers which need this sort of adaptive
> > allocation isn't too high and the common cases can be made simple, I
> > don't think the "complex" part of interface is all that important.
> > Maybe we can have reserve / cancel type interface or just keep the
> > loop with more explicit function names (ie. try_enable or something
> > like that).
>
> I'm thinking a better API overall might just have been to request
> individual MSI-X one by one :-)
>
> We want to be able to request an MSI-X at runtime anyway ... if I want
> to dynamically add a queue to my network interface, I want it to be able
> to pop a new arbitrary MSI-X.

Yes, this would be very useful.

> And we don't want to lock drivers into contiguous MSI-X sets either.

I don't think there's any such limitation now. The entries array passed
to pci_enable_msix() specifies which MSI-X vectors the driver wants to
enable. It's usually filled with 0..nvec-1 in order, but not always.
And the IRQ numbers returned aren't usually contiguous either, on x86.

Ben.

> And for the cleanup ... well that's what the "pcim" functions are for,
> we can just make MSI-X variants.

--
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.